Payson may have secured its long-term water supply just in time.
Recent studies suggest that the Southwest continues to use about double the long-term water supply, given projections of climate shifts and a resulting increase in the length and intensity of droughts.
Payson last week started construction on preliminary phases of the Blue Ridge pipeline, which by 2014 will start to deliver about 3,000 acre-feet annually from a reservoir atop the Mogollon Rim.
That water will more than double the town’s long-term water supply, providing enough water to triple the town’s population while also accommodating a sought-after 6,000-student university campus and related facilities.
The town’s roughly 16,000 residents currently use about 1,800 acre-feet of water annually. Payson hopes to initially pump water from the Blue Ridge Reservoir into the town’s underground water table, which has fallen more than 100 feet in recent decades.
The start of construction on the pipeline comes just as a mounting array of studies suggests that deepening drought will in coming decades drain the enormous reservoirs on the Colorado River that provide water for some 30 million people in the region.
Projected global warming trends will likely mean that in the long-term water users in seven western states can count on only 40 percent of the water in the Colorado River Basin. However, the region currently uses about 76 percent of the water in the system each year — roughly double the sustainable benchmark, according to a study by Arizona State University School of Life Sciences associate professor John Sabo, who also serves as director of research development in ASU’s Global Institute of Sustainability.
The vast reservoirs along the Colorado River, including Lake Mead, Lake Powell and Lake Havasu, currently store a roughly five-year supply of water for users in California, Arizona, New Mexico, Colorado, Utah, Nevada and Wyoming. Most of the water goes to Colorado, but Phoenix and Tucson both depend on water diverted into the multi-billion-dollar Central Arizona Project.
Lake Mead holds the bulk of the water in storage, but an intermittent decade-long drought recently drained it to below half its capacity, a record low.
Sabo said the Southwest must cut its water use by about 60 percent to bring water supplies into balance, given projections of longer, deeper droughts in the decades ahead as average global temperatures rise with the build-up of heat-trapping gases like carbon dioxide and methane.
The recent projections echo an earlier study by researchers from the University of Colorado at Boulder, which said the Colorado River reservoirs have a 50 percent chance of essentially running dry in the next half century.
The chain of reservoirs has such an enormous storage capacity that despite climate models showing more frequent drought conditions the odds of running out of water in the Colorado River system amount to no more than 10 percent through 2026. However, even a 10 percent decline in average rainfall would spell mounting trouble in coming decades.
The study concluded that given a projected 20 percent reduction in average rainfall included in many climate models, the odds the Colorado Reservoirs will go dry by 2057 rise to about 50 percent — even without any increase in water use.
Payson this year has received about half its normal rainfall — roughly 8 inches compared to the long-term average of 16 inches at this point in the year. The region suffered through a bone-dry spring and a relatively normal monsoon, but still heads into the winter with depleted reservoirs and parched forests.
The Blue Ridge Reservoir can store about 14,000 acre-feet. Payson has a legal entitlement to about 3,000 acre-feet and other Northern Gila County communities to about 500 acre-feet. However, few other communities in the region have contracted with the Salt River Project to get a share of that Blue Ridge water.
The giant reservoirs on the Colorado River can hold about 60 million acre-feet, about four times the river’s average annual flow.
The region could cope with a big drop in its water supply by curtailing agriculture, which consumes about 77 percent of the water used.
For instance, farms in the Southwest grow about 75,000 acres of thirsty lettuce annually, most of it near Yuma, which has an annual rainfall tally of 3.6 inches in a good year. By contrast, Payson gets about 22 inches of rain annually.
The plumbing system originally designed to deliver irrigation water to Valley farms has largely sustained urban growth in the Valley in recent decades. Maricopa County’s population grew by 24 percent from 2000 to 2010, while Gila County’s population barely budged. The U.S. Census Bureau estimates the Phoenix-Tucson metropolitan area’s population will increase by about 50 percent by 2030, to nearly 8 million.